This invention relates to a multitubular heat exchanger, and more particularly to a feedwater heater for preheating feedwater supplied to a turbine plant where steam is used as driving power source.
There is an increasing tendency toward enlargement of the feedwater heater system to keep line with ever-expanding power plant capacity, but a request is now voiced in the industry for innovation of the structural system in view of the limitations in production techniques, construction, function and so forth.
Among the single-body type feedwater heaters of the prior art is the device disclosed in U.S. Pat. No. 3,020,024 (patent granted Feb. 6, 1962). In this feedwater heater, both ends of the heating tubes bent in U shape and arranged in the inside of the heater body are disposed in a hemispherical water chamber formed integral with the tube plate. In the inside of said water chamber is provided a partition plate which defines a feedwater inlet and a feedwater outlet. Provided in this feedwater heater are a desuper heating zone, a condensing zone and drain subcooling zone, and the feedwater supplied into the heater from the inlet is heated while flowing through the heating tubes by hot steam also flowing in the heater body, and is discharged out from the outlet as highly heated water. On the other hand, hot steam supplied from a steam inlet enters first into the desuper heating zone, then flows into the condensing zone for condensation therein, and then further advances into the drain subcooling zone where steam condensate is cooled and then discharged out from the condensate outlet. The desuper heating zone is defined by an inner cylinder disposed so as to cover the corresponding heating tube portions, while the drain subcooling zone is also defined by an inner cylinder which is also so disposed as to sheathe the corresponding heating tube portions.
The feedwater heaters of such conventional system incorporating all the component elements in one body have a drawback that the entire heater unit is enlarged in size as the heat transfer area in the heater is increased with expansion of the plant capacity and also the thickness of the shell as well as the tube plates of the water chambers must be increased to withstand the elevated steam pressure. They also involve the problem that great difficulties attend in machining holes for inserting the heating tube ends owing to increased thickness of the tube plate.
Among the proposed heat exchangers with enlarged structure is known a hairpin type heat exchanger such as disclosed in U.S. Pat. No. 3,249,153. Adaptation of this heat exchanger as a feedwater heater for a turbine plant, however, is attended by difficulties in installation because it is required for providing sufficient strength to enlarge the size while increasing thickness of both the inner cylinder defining the desuper heating zone and the inner cylinder defining the drain subcooling zone. Further, as the joints of the two hairpin-forming body portions are of a flange type, difficulty is encountered in sealing the high-pressure fluid flowing in the system, and also the body structure is enlarged because of necessarily increased flange thickness.